Among the many issues being discussed by the scientists, providers, and advocates gathered at the 2014 Conference on Retroviruses and Opportunistic Infections (CROI) in Boston this week is pre-exposure prophylaxis (PrEP) for HIV. In a number of clinical trials, daily oral PrEP has been proven to be an effective biomedical intervention for reducing the risk of HIV acquisition among adult men and women at very high risk for HIV infection through sex or injecting drug use. While studies have demonstrated the efficacy of PrEP and the Food and Drug Administration (FDA) has approved the use of Truvada® as PrEP, questions remain regarding how it can best be deployed to prevent HIV transmission in real world settings in the U.S. and elsewhere around the world.

Researchers are sharing their findings in various sessions across the conference, examining issues ranging from interest in and acceptability of PrEP among various populations, to factors that influence uptake and adherence, and strategies for deploying PrEP in the U.S. and in resource-limited settings in other parts of the world. Some of the major themes include:

Adherence is essential

Clinical pharmacology studies discussed at the conference have helped explain the varied range of outcomes across PrEP trials – some of which have demonstrated very high protection in some populations and no benefit in others. In a PrEP symposium on Tuesday, March 4, a researcher noted that highly variable adherence to the PrEP medication across studies explains much of the outcome variation. “There is a clear concentration-response relationship between tenofovir and its protective benefit that indicates PrEP regimens are highly effective, if taken as directed,” noted Dr. Craig Hendrix of The Johns Hopkins University School of Medicine. He noted, however, that the number of doses required to achieve an effective concentration for the desired protective effect in an individual’s body remains a question of study and that it is possible that different concentrations may be required for men and women.

Innovative implementation strategies are key – Clinical trial efficacy is only the first step towards public health effectiveness, observed Dr. Nelly R. Mugo of the Kenya Medical Research Institute in Nairobi, also during that symposium. To translate the promise of PrEP from clinical trial efficacy to public health effectiveness, PrEP requires innovative implementation strategies, she noted. Dr. Mugo stressed that PrEP, and the way it is delivered, must be both acceptable and accessible to the populations that most need it. She suggested that in some resource-constrained settings PrEP might be used as a “bridge to ART;” since not all HIV positive partners in serodiscordant couples will choose to or can start antiretroviral therapy (ART) immediately, initiating PrEP with the HIV-negative partner provides discordant couples with protection against HIV transmission until the HIV-positive partner begins ART and achieves a suppressed viral load.

Understanding influences on uptake

Given their disproportionate and rising burden of HIV in the U.S., young men who have sex with men (YMSM), particularly racial/ethnic minority YMSM, are an anticipated target population for PrEP interventions. Another of the presentations during the symposium discussed efforts in Chicago to engage YMSM between the ages of 15 and 22 in a PrEP trial supported by the NIH’s Adolescent Trials Network. According to Mr. Christopher Balthazar from Stroger Hospital of Cook County in Chicago, recruitment efforts have revealed a dearth of knowledge in communities about this new HIV prevention approach. He noted that they encountered not only lack of community knowledge, but also varying degrees of community support. Clearly these perspectives are among the obstacles that need to be addressed as other efforts begin to target PrEP to populations at high risk of HIV infection. Mr. Balthazar explained that among the concerns the study team encountered from participants were concerns about not getting protection if they missed a dose and concerns about side effects. On the other hand, among the reasons for initiating PrEP cited by the participants were a desire to have back-up protection when condoms are not being used, being in a serodiscordant relationship, fear of partner infidelity, and experience being unsuccessful with use of condom from start to finish. Understanding both concerns and motivators among target populations will be vital to the success of future efforts to deploy PrEP effectively.

Next generation PrEP

Finally, elsewhere at the conference we learned that CDC is conducting animal studies to evaluate the next generation of PrEP options, including injectable delivery methods and vaginal gels. According to CDC’s Dr. Gerardo Garcia-Lerma, monthly injections with a long-acting formulation of the HIV integrase inhibitor GSK744 completely prevented vaginal simian/human immunodeficiency virus (SHIV) infection in pigtail macaques. PrEP with long-acting drug formulations has the potential to overcome challenges of adherence associated with taking a pill daily. The Dr. Garcia-Lerma and his fellow study authors concluded that the data show promise for advancement to human trials of the long lasting formulation of GSK744.

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